High-speed underwater communication with GaN blue LDs (Conference Presentation)

Abstract

To enable high-speed underwater wireless optical communication (UWOC) in tap-water and seawater environments over long distances, a 450-nm blue GaN laser diode (BLD) directly modulated by preleveled 16-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) data was employed to implement its maximal transmission capacity beyond 10 Gbps. The proposed UWOC in tap water provided a maximal allowable communication bitrate increase from 5.2 to 12.4 Gbps with the corresponding underwater transmission distance significantly reduced from 10.2 to 1.7 m. Light scattering induced by impurities in seawater attenuates the blue laser power to degrade the transmission such that the BLD based UWOC enables a 16-QAM OFDM bitrate of up to 7.2 Gbps in seawater more than 6.8 m. To optimize the QAM-OFDM transmission, the sampling rate of the encoded data is compromised to avoid the aliasing and oversampling effects during waveform extraction procedure. The sampling rate is optimized to 3~5 times of the encoded data bandwidth for suppressing peak-to-average power ratio (PAPR). This improves the BLD based UWOC with 16-QAM OFDM data transmission at 14.8 Gbps over 1.7 m. Lengthening the seawater distance to 10.2 m only decreases the transmission data rate by 4 Gbps, Oversampling not only filters out background noise but also attenuates data amplitude to degrade transmission performance. With spectrally filtering out the sidelobes of each OFDM subcarrier, the allowable modulation bandwidth is greatly improved from 1.9 to 2.7 GHz, as the inter-carrier interference induced crosstalk between subbands is relieved to improve the SNR of the carried data with a raw data rate of up to 10.8 Gbps over 10.2 m in seawater channel.